This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/FI98/00866 which has an International filing date of Nov. 9, 1998, which designated the United States of America.
1. Field of the Invention
The present invention relates to propylene polymers. In particular, the present invention concerns a process for preparing efficiently nucleated propylene copolymers.
2. Description of Related Art
Propylene (PP) homo- and copolymers have excellent resistance to heat and chemicals as well as attractive mechanical properties, such as stiffness and impact resistance. However, processing of polypropylene by, e.g., injection moulding, thermoforming or blow moulding, to form thin-walled containers has resulted in products having insufficient stiffness, transparency and cycle time. This is caused by the semi-crystalline nature of polypropylene.
In the prior art it has been proposed to improve the stiffness, transparency and cycle time of moulded polypropylene by blending the polymer with various nucleating agents such as dibenzilidene sorbitol (DBS), sodium benzoate or di(alkylbenzilidene)sorbitol. These traditional nucleating agents tend to bleed out from the polymer composition during processing and many of them give rise to fumes with an offensive smell. As a solution to these problems, it has been suggested in the art to use vinyl compounds, such as polymers of vinyl cycloalkanes and 3-methyl-1-butene, as nucleating agents in the form of propylene copolymers or polypropylene compounds, cf. EP Patent Specifications Nos. 0 151 883, 0 152 701, 0 206 515, 0 368 577 0 369 658 and 0 417 319. EP Patent No.0 152 701 discloses prepolymerization of Ziegler-Natta catalysts with vinyl cyclohexane to provide a polymer slurry which is washed and distilled to produce a vinyl cyclohexane powder containing the active catalyst. The prepolymerized catalyst composition is then used for polymerization of propylene to form a propylene copolymers with improved stiffness and having a high degree of crystallinity and a high crystallization temperature.
There are some major problems associated with the prior art solutions using polymerized vinyl compounds for nucleation of polypropylene. Thus, the products contain impurities in the form of unreacted monomers and extensive washing of the product has to be carried out before the catalyst can be used. These washing steps will reduce that activity of the catalyst. In fact, the whole work-up of the prepolymerized catalyst, including separation of the catalyst from the polymerization medium, washing and drying, will cause extra costs and impair the activity of the catalyst.
Finally, it should be pointed out that it is known in the art to carry out prepolymerization using, e.g. vinyl cyclohexane, in a medium comprising a viscous substance (cf. Finnish Patent No. 95387). Due to the fact that, e.g., no washing, drying, sieving and transferring steps are needed, the catalyst activity is maintained.
It is an object of the present invention to eliminate the problems related to the prior art and to provide a novel process for preparing nucleated propylene polymers, i.e. propylene homopolymers, propylene copolymers and propylene random block polymers. In particular the present invention aims at improving the prior art of Finnish Patent No. 95387 (corresponds to EP Patent 0 607 703) while providing a process by which it is possible to prepare modified catalysts providing excellent nucleation of propylene polymers and containing essentially no reactant or solvent residues which would impair the long-term activity of the catalysts.
Further, it is an object of the present invention to provide a process for preparing propylene polymers which can be used in food contact packaging applications due to the fact that they do not contain any residues of taste- or odor-components.
It is still a further object of the present invention to provide extruded and moulded products comprising the present propylene homo- and copolymer compositions.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The invention is based on providing a catalyst composition useful for polymerization of propylene optionally together with comonomers. The catalyst is modified by polymerizing a vinyl compound in the presence of said catalyst. The modification takes place in the medium in which the catalyst also is fed into the polymerization process. The medium is a liquid or a highly viscous hydrocarbon medium which does not dissolve the polymerized polymer. Further, the modification is accomplished in such a way that all or practically all of vinyl compound used for modification is consumed during polymerization. To reach that aim, the polymerization is continued at elevated temperature until the concentration of the unreacted vinyl compounds in the catalyst composition after polymerization is about 0.5 wt-%, preferably less than 2000 ppm by weight and in particular 1000 ppm by weight or less. The initial amount of reactant is also restricted to a maximum of up to three times the weight of the catalyst. The modification is carried out before any conventional, usually continuous prepolymerization with an olefinic monomer, to ensure that the polymerization of the vinyl compound is complete.
As a result of these features, the amount of reactant residues in the modified catalyst composition is small, and in the final polymer it is below limits of determination using the Gas Chromatography-Mass Spectrometry (GC-MS) method, which is less than 0.01 ppm by weight. Since the reaction medium does contain only very small amounts of unreacted reactant residues or dissolved polymer residues, no washing of the modified catalyst composition is needed before the catalyst is fed to polymerization.
More specifically, the invention is mainly characterized by a process for producing a propylene polymer nucleated with a polymeric nucleating agent containing vinyl compound units, comprising the steps of modifying a catalyst by: polymerizing a vinyl compound of the formula 
wherein R1 and R2 together from a 5 or 6 membered saturated or unsaturated or aromatic ring at a weight ratio of the vinyl compound to the catalyst amounting to 0.1 to below 3, in the presence of said catalyst, by carrying out the modification in a medium which does not essentially dissolve the polymerized vinyl compound and by continuing the polymerization of the vinyl compound until the concentration of unreacted vinyl compounds is less than 0.5 wt-%, to produce a modified catalyst composition, said modification being carried out essentially before any prepolymerization step of the catalyst with an olefinic monomer, and polymerizing propylene optionally together with comonomers in the presence of said modified catalyst composition.
The invention achieves a number of considerable advantages. Modification of the catalyst by using it for polymerization of vinyl compounds in the liquid or highly viscous medium described above will reduce production costs because higher capacities can be used and no wastes are formed. Reliability of the catalyst activity is good because this modification is a part of the polymer production and no kind of transferring, drying or sieving is needed.
Because the final products do not contain harmful residues of the vinyl compounds, the propylene polymers manufactured by the present invention have a broad range of application.
Although the use of certain types of external donors will yield interesting and valuable effects on the polymers produced, as explained in our copending patent application, by means of the present invention it is also possible to produce a modified catalyst composition with high and enduring activity without any external donor.
By using the modified catalyst compositions of the present invention, propylene polymers can be prepared having a Melt Flow Rate (MFR2) of 0.01 to 1000 g/10 min (measured by ISO Standard 1133, at 230xc2x0 C., 2.16 kg load) and a Tcr of over 7xc2x0 C. higher than the Tcr of the corresponding non-nucleated polymer. The crystallinity of propylene homopolymers is generally over 48%.
The present process can be employed for producing propylene polymers which are useful in many different kinds of polymer articles. Propylene polymers produced according to the invention can be used in pelletized or non-pelletized form as will be discussed later in this application. Particular advantages are obtained by applying the polymers preferably blended or compounded with other components for the manufacture of moulded and extruded products.
Next, the invention will be more closely examined with the aid of the following detailed description with reference to the attached drawing.